It is well known in the art of semiconductor packaging to bond a semiconductor chip to a metal substrate, such a substrate typically being selected for its high coefficient of thermal conductivity and its corresponding ability to function as a heat sink. A layer of soft solder is typically used to bond the chip to the metal substrate, thus forming a semiconductor chip package. Such a package conducts heat from the semiconductor chip through the solder layer to the metal substrate, thereby maintaining the chip at an acceptable operating temperature.
A well recognized, though undesirable phenomenon associated with this type of semiconductor chip packaging is the failure of the solder layer bond between the chip and the substrate. Such failure is due to a thermal fatigue induced in the solder layer due to a large difference in the values of the coefficient of thermal expansion for the metal substrate versus the semiconductor chip. The metal substrate, selected for its high coefficient of thermal conductivity, typically exhibits a correspondingly high coefficient of thermal expansion. A semiconductor chip, however, typically exhibits a very low coefficient of thermal expansion. Thus, when the semiconductor chip package is subjected to thermal cycling normally associated with the operation of the semiconductor chip, the metal substrate will exhibit large amounts of planar expansion and contraction, while the semiconductor chip will remain substantially the same size. During this thermal cycling, the solder layer, disposed intermediate the chip and the substrate, undergoes substantial stresses and strains. These stresses and strains cause the solder layer bond to fail, thereby eventually rendering the semiconductor chip inoperable. Much effort has recently been directed to solving this problem of thermal fatigue in the solder layer bond by altering the metallurgical properties of either the metal substrate or the solder layer bond. To date, these efforts have yielded results that are either prohibitive in cost or ineffective in solving the problem.